1. Field of the Invention
The present invention relates to a coil component used as a main component of a common mode choke coil or a transformer and a method of manufacturing the same.
2. Description of the Related Art
Reductions in the size of electronic apparatus such as personal computers and portable phones have resulted in demand for reductions in the size and thickness (low height) of electronic components such as coils and capacitors mounted on internal circuits of electronic apparatus.
However, a wire-wound coil obtained by winding a copper wire or the like around a ferrite core has a problem in that it is difficult to make compact because of structural limitations. Under the circumstance, research and development is active on chip-type coil components which can be provided with a small size and a low height. Known chip-type coil components include multi-layer type coil components provided by forming coil conductor patterns on surfaces of magnetic sheet made of ferrite or the like and stacking the magnetic sheets and thin film type coil components provided by forming insulation films and coil conductors constituted by metal thin films alternately using thin film forming techniques.
Patent Documents 1 to 3 disclose common mode choke coils that are thin film type coil components. FIG. 10 is a sectional view of a common mode choke coil 51 taken along a plane including center axes of coil conductors 59 and 61. As shown in FIG. 10, the common mode choke coil 51 has an insulation layer 57 formed by stacking an insulation film between ferrite substrates (magnetic substrates) 53 and 55 which are provided opposite to each other. The coil conductors 59 and 61, which are provided opposite to each other with the insulation film interposed between them and formed in a spiral configuration, are embedded in the insulation layer 57. The insulation layer 57 and the coil conductors 59 and 61 are formed in the order listed using thin film forming techniques.
An open region 63 is formed on an inner peripheral side of the coil conductors 59 and 61 having a spiral configuration by removing the insulation layer 57. An open region 65 is formed on an outer peripheral side of the spiral coil conductors 59 and 61 by removing the insulation layer 57. Magnetic layers 67 are formed to fill the open regions 63 and 65. Further, a bonding layer 69 is formed on the magnetic layers 67 and the insulation layer 57 to bond a magnetic substrate 55.
When the coil conductors 59 and 61 are energized, a magnetic path M is formed such that it passes through the magnetic substrate 53, the magnetic layer 67 in the open region 63, the bonding layer 69, the magnetic substrate 55, the bonding layer 69 again and the magnetic layer 67 in the open region 65 in the section including the center axes of the coil conductors 59 and 61. The bonding layer 69 is a film having a thickness on the order of a few μm, although it is non-magnetic. Therefore, substantially no leakage of the magnetic flux occurs in this part, and the magnetic path M may be regarded as a substantially closed path.
In order to improve differential transmission (balanced transmission) characteristics of the common mode choke coil 51, a capacitance (stray capacitance) C which is generated between the coil conductors 59 and 61 must be made small. The capacitance C is parasitically generated in parallel with inductances of the coil conductors 59 and 61. Therefore, when a relatively high capacitance C is generated, the capacitance C will dominantly constitute the impedance of the common mode choke coil 51 in high frequency bands. Since the impedance constituted by the capacitance C is inversely proportionate to a frequency, the impedance of the common mode choke coil 51 decreases, which results in degradation of differential transmission characteristics.
The capacitance C between the coil conductors 59 and 61 can be expressed by C=ε×(S/d) where d represents the inter-layer distance between the coil conductors 59 and 61; S represents the area in which the conductors face each other; and ε represents the dielectric constant of the region between the coil conductors 59 and 61 (the dielectric constant of the insulation layer 7). Since section of the coil conductors 59 and 61 are formed with a rectangular configuration, the area S over which the coil conductors 59 and 61 face each other is relatively large. Further, the coil conductors 59 and 61 are formed such that the inter-layer distance d between them becomes very small to provide the common mode choke coil 51 with a low height and to maintain predetermined common mode filter characteristics. As a result, a relatively high capacitance C is generated between the coil conductors 59 and 61, and differential transmission characteristics are therefore degraded.
Patent Document 4 discloses a pair of coils disposed in a face-to-face relationship and having a sectional configuration in which corners of the coils are rounded. In comparison to coils having a rectangular section like the coil conductors 59 and 61, the area over which the coils having a sectional configuration with rounded corners face each other at a minimum inter-layer distance between them is smaller, therefore a capacitance generated between the upper and the lower coils become a slightly lower. However, since the area over which the upper and the lower coils face each other at a minimum inter-layer distance is still relatively large in spite of the fact that the coil section has rounded corners, the differential transmission characteristics of the coils cannot be sufficiently improved.
Patent Documents 5 to 7 disclose sectional configurations of a pair of coils provided in a face-to-face relationship in a thin film magnetic head. Surfaces of such coils facing each other are curved or formed in a trapezoidal configuration when viewed in their sections. Since the purpose of such sectional configurations is to achieve effects such as a reduction in the magnetic path length between the magnetic poles of a thin film magnetic head, conductive parts of the upper and lower coils are interleaved between each other, and there are fundamental structural differences between those coils and the common mode choke coil 51, including a difference in wiring between upper and lower coils, i.e., series and parallel wiring.
Patent Document 1: Japanese Patent Laid-Open No. JP-A-2003-133135
Patent Document 2: Japanese Patent Laid-Open No. JP-A-11-54326
Patent Document 3: Japanese Patent Application No. 2003-307372
Patent Document 4: Japanese Patent No. 2011372
Patent Document 5: Japanese Patent No. 2677415
Patent Document 6: Japanese Patent Laid-Open No. JP-A-2000-182213
Patent Document 7: Japanese patent No. 3086212
To reduce the height of the common mode choke coil 51 and maintain predetermined common mode filter characteristics, the inter-layer distance d between the coil conductors 59 and 61 must be reduced. As a result, a relatively high capacitance C is generated between the coil conductors 59 and 61, which results in a problem in that the differential transmission characteristics cannot be sufficiently improved.